Welding bands are utilized in equipment for the welding of synthetic resin film as is for instance described in European patent publication 0 155 548 B1. In this equipment, continuous webs of film, for example tubular webs, are welded into bags. The correspondingly folded film web is conveyed until the point to be welded is located between suitable welding jaws. The welding band is arranged on the welding jaws and is in the form of a resistance heating element. The welding band is heated and the film web to be welded is clamped between the opposed welding jaws. The heat of the welding band is transmitted to the film so that it is welded.
The welding band consists of a narrow steel band or tape. A TEFLON band extends between the welding band and the film web to be welded. This TEFLON band extends over the entire welding jaw, that is to say beyond the edge of the film web to be welded. If the entire welding band is manufactured of steel, there will be local overheating in the edge portion of the tubular film web so that there may be damage to the TEFLON bands or even short-circuiting. As a remedy in this respect it has already been proposed to coat the end parts of the welding band with a thin copper layer, which provides satisfactory thermal and electrical conductivity. Owing to this improved thermal conductivity local overheating of the welding band is prevented.
If such welding bands are utilized for weldingly sealing bags filled with corrosive substances, as for instance salts, there is the problem that after a relatively short period of use, the copper layer is lost owing to corrosion. This in turn leads to local overheating of the welding band and, as a result, damage to the TEFLON tape.